I chose the SL MKIII over the S61 as a second keyboard and because it has more controls, but more importantly because you can control the displays (which you can’t really do with the SI61).
If the NI S61 gave the ability to fully control the displays I probably would have gone that route. But NI shows no interest in making screen control available. They’re nice if you’re content to use them exclusively with NI’s software, but they don’t do much for use with GigPerformer.
The SL MKIII display and buttons are fully accessible through a programming API. I’ve written a GigPerformer extension for it (which I’ll make freely available if you or anyone else wants to use or modify it. It’s up on Github, but I still have it tagged as private.)
The picture below is probably hard to make out very well, but I have the displays showing GP parameter names on each of the 8 knobs, the knob position, and the text value (e.g., -2db or whatever). The arrows to the left of the display can “bank switch” through however many banks of knobs you feel like creating for each rackspace. I have the buttons underneath the displays set up to select Rackspace/Variation/Song/Songpart and you can scroll through those with a different set of arrow keys. The names of the Racks/Variations, etc. are on the bottom lines of the display right above the buttons.
I find the keybed to be fine for semi-weighted synth style keys.
